Control systems use discrete inputs and outputs (I/O) to communicate between devices, to pass information such as status or to issue commands. In comparison to communications networks, discrete I/O is seen as a less complex and costly solution in many applications that require simple yet reliable operation. Of particular interest is the design of binary input ports since these are probably the most widely used input port types in control equipment. These input ports must be designed to differentiate between at least two specific signal levels.
FIG. 1 illustrates a simplified view of a discrete input/output circuit 10 of the prior art in which a remote device 14 is monitored by a binary input port identified in FIG. 1 as a logic device 18. The remote device 14, logical device 18 and a power supply 22 are connected electrically in series by a first conductor 26 connected between a first terminal 30 of the power supply 22 and first terminal 34 of the remote device 14, a second conductor 38 connected between a second terminal 42 or the remote device 14 and a first terminal 46 of the logic device 18, and a third conductor 50 connected between a second terminal 54 of the logic device 18 and a second terminal 58 of the power supply 22. The remote device 14 consists of a two-state switch S1 (either electromechanical or solid-state) that when open interrupts the flow of power from a power supply 22 to the logic device 18 and when closed, completes the circuit path between the power supply 22 and the logic device 18. The logic device 18 senses the presence or absence of voltage from the power supply 22 across terminals 46 and 54 of the logic device 18 and translates these voltage levels into logic signals for use by a control system.
For the following discussions, when the state of switch S1 in the remote device 14 is closed and voltage from the power supply 22 is present across the logic device 18 input terminals 46 and 54 this state will be referred to as the “ON state” or “active state” of the circuit 10 or logic device 18. When the state of switch S1 of the remote device 14 is open and voltage from the power supply 22 is not present across the terminals 46 and 54 of logic device 18 will be referred to as the “OFF state” or “inactive state” of the circuit 10 or logic device 18.
Since the logic device 18 has impedance associated with the path between its terminals 46 and 54, when the remote device 14 switch S1 closes, a current will flow around the circuit 10 consisting of the power supply 22, remote device 14 and logic device 18. The impedance of the overall circuit 10 and the power supply 22 voltage magnitude will determine the current amplitude and other characteristics. For the circuit 10 shown in FIG. 1, the power supply 22 can be either AC or DC.
The ability to differentiate signal levels is made difficult by the presence of electrical noise found in the operating environment of the system 10. In FIG. 1, the remote device 14 can be a considerable distance from the logic device 18 and the interconnecting conductors 26 and 38 are subjected to local electrical field phenomenon known as electrical noise. Many forms of electrical noise exist that can become entwined with the received signals thereby corrupting the signal integrity. Induction and capacitive coupling of signals or impulses from adjacent circuits is possible. Additionally, current leakage (either galvanic or displacement) across or through insulation of the conductors 26 and 38 or devices 14 and 18 represent a form of electrical interference sometimes classified as noise. Other forms of electrical noise can be present and are well known to those skilled in the art.
Referring now to FIG. 2, a large classification of electrical noise can be represented by a Thevenin equivalent circuit 62 attached between conductors 26 and 38 and effectively shunts the remote device 14. The Thevenin voltage source can represent noise signals of various types (periodic, non-periodic), waveshapes (sinusoidal, impulsive, rectangular, etc.), frequencies or amplitudes. The Thevenin voltage source provides the driving energy to disturb the circuit. The associated Thevenin impedance determines how tightly coupled are the noise signals to the circuit 10. For a Thevenin noise model, if the impedance of the logic device 18 is equal to the Thevenin impedance and the Thevenin noise voltage is of the same magnitude and polarity of the power supply 22, a voltage equivalent to the power supply 22 voltage will be applied across the logic device 18 input terminals 46 and 54. This voltage will cause the logic device 18 to report an incorrect ON or OFF state of the remote device 14 switch S1.
From a system perspective, proper selection of the operational signal levels, signal detection thresholds, input impedance and response time all determine the effectiveness of the logic device 18 to reliably receive information in the presence of the environmental electrical noise. For instance, if the impedance across the logic device 18 was small compared to the Thevenin impedance (say 1/100 of ZT), then only 1/100 of the Thevenin noise voltage would be present across the logic device 18 input terminals 46 and 54. Such a reduction in noise voltage greatly increases the ability of the logic device 18 to resolve the correct ON or OFF state of the remote device 18 switch S1.
Reducing the impedance of the logic device 18 with respect to the noise source impedance while very effective, does have limitations. The lower the impedance, the more current will flow around the circuit 10 when the remote device 14 switch S1 is closed. The increased current places a greater power burden on the power supply 22 and causes additional heat dissipation in both the power supply 22 and logic device 18. Therefore, it becomes desirable to set the logic device 18 impedance as high as possible while still minimizing the effects of noise on the state of the circuit 10. The desirability of attaining this goal increases greatly as the number of logic devices 18 present increases.
Additionally, the designer must deal with a number of other constraints in the logic devices 18. These include but are not limited to internal heating, channel density, signal integrity issues, speed, and cost. These constraints are normally in direct opposition to each other. For example, simple designs will allow for a low cost, but can cause an unacceptably large power dissipation requiring additional methods to remove the wasted heat; while low power designs minimize the heating issues but require higher part counts.